Re: CFQ and dm-crypt

[Milan Broz <mbroz@redhat.com>]

On 10/24/2010 03:51 PM, Richard Kralovic wrote:
> CFQ io scheduler relies on using task_struct current to determine which
> process makes the io request. On the other hand, some dm modules (such
> as dm-crypt) use separate threads for doing io. As CFQ sees only these
> threads, it provides a very poor performance in such a case.
> 
> IMHO the correct solution for this would be to store, for every io
> request, the process that initiated it (and preserve this information
> while the request is processed by device mapper). Would that be feasible?

Yes, this seems to be correct solution. I think this should be
handled by core device-mapper (as you noted, more dm targets using
threads to process.)


> Other possibility is to avoid using separate threads for doing io in dm
> modules. The attached patch (against 2.6.36) modifies dm-crypt in this
> way, what results into much better behavior of cfq (e.g., io priorities
> work correctly).

Sorry, this completely dismantles the way how dm-crypt solves problems
with stacking dm devices.
Basically it reintroduces possible deadlocks for low memory
situations (the reason why there are these threads).

Milan